Combination Of Angle-resolved Photoemission Spectroscopy Research Articles

On-Surface Synthesis of Polyene-Linked Porphyrin Cooligomer.

π-Conjugated molecules are viewed as fundamental components in forthcoming molecular nanoelectronics in which semiconducting functional units are linked to each other via metallic molecular wires. However, it is still challenging to construct such block cooligomers on the surface. Here, we present a synthesis of [18]-polyene-linked Zn-porphyrin cooligomers via a two-step reaction of the alkyl groups on Cu(111) and Cu(110). Nonyl groups (-C9H19) substituted at the 5,15-meso positions of Zn-porphyrin were first transformed to alkenyl groups (-C9H10) by dehydrogenation. Subsequently, homocoupling of the terminal -CH2 groups resulted in the formation of extended [18]-polyene-linked porphyrin cooligomers. The structures of the products at each reaction step were investigated by bond-resolved scanning tunneling microscopy at low temperatures. A combination of angle-resolved photoemission spectroscopy and density functional theory calculations revealed the metallic property of the all trans [18]-polyene linker on Cu(110). This finding may provide an approach to fabricate complex nanocarbon structures on the surface.

Weyl nodal ring states and Landau quantization with very large magnetoresistance in square-net magnet EuGa4

Magnetic topological semimetals allow for an effective control of the topological electronic states by tuning the spin configuration. Among them, Weyl nodal line semimetals are thought to have the greatest tunability, yet they are the least studied experimentally due to the scarcity of material candidates. Here, using a combination of angle-resolved photoemission spectroscopy and quantum oscillation measurements, together with density functional theory calculations, we identify the square-net compound EuGa4 as a magnetic Weyl nodal ring semimetal, in which the line nodes form closed rings near the Fermi level. The Weyl nodal ring states show distinct Landau quantization with clear spin splitting upon application of a magnetic field. At 2 K in a field of 14 T, the transverse magnetoresistance of EuGa4 exceeds 200,000%, which is more than two orders of magnitude larger than that of other known magnetic topological semimetals. Our theoretical model suggests that the non-saturating magnetoresistance up to 40 T arises as a consequence of the nodal ring state.

Quantum Confinement and Electronic Structure at the Surface of van der Waals Ferroelectric α-In2Se3.

Two-dimensional (2D) ferroelectric (FE) materials are promising compounds for next-generation nonvolatile memories due to their low energy consumption and high endurance. Among them, α-In2Se3 has drawn particular attention due to its in- and out-of-plane ferroelectricity, whose robustness has been demonstrated down to the monolayer limit. This is a relatively uncommon behavior since most bulk FE materials lose their ferroelectric character at the 2D limit due to the depolarization field. Using angle resolved photoemission spectroscopy (ARPES), we unveil another unusual 2D phenomenon appearing in 2H α-In2Se3 single crystals, the occurrence of a highly metallic two-dimensional electron gas (2DEG) at the surface of vacuum-cleaved crystals. This 2DEG exhibits two confined states, which correspond to an electron density of approximately 1013 electrons/cm2, also confirmed by thermoelectric measurements. Combination of ARPES and density functional theory (DFT) calculations reveals a direct band gap of energy equal to 1.3 ± 0.1 eV, with the bottom of the conduction band localized at the center of the Brillouin zone, just below the Fermi level. Such strong n-type doping further supports the quantum confinement of electrons and the formation of the 2DEG.

Electronic Instability of Kagome Metal CsV3Sb5 in the 2 × 2 × 2 Charge Density Wave State

Recently discovered kagome metals AV3Sb5 (A = K, Rb, and Cs) provide an ideal platform to study the correlation among nontrivial band topology, unconventional charge density wave (CDW), and superconductivity. The evolution of electronic structures associated with the change of lattice modulations is crucial for understanding of the CDW mechanism, with the combination of angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory calculations, we investigate how band dispersions change with the increase of lattice distortions. In particular, we focus on the electronic states around point, where the van Hove singularities are expected to play crucial roles in the CDW transition. Previous ARPES studies reported a spectral weight splitting of the van Hove singularity around point, which is associated with the 3D lattice modulations. Our studies reveal that this “splitting” can be connected to the two van Hove singularities at kz = 0 and kz = π/c in the normal states. When the electronic system enters into the CDW state, both van Hove singularities move down. Such novel properties are important for understanding of the CDW transition.

Spontaneous Ferromagnetism Induced Topological Transition in EuB_{6}.

The interplay between various symmetries and electronic bands topology is one of the core issues for topological quantum materials. Spontaneous magnetism, which leads to the breaking of time-reversal symmetry, has been proven to be a powerful approach to trigger various exotic topological phases. In this Letter, utilizing the combination of angle-resolved photoemission spectroscopy, magneto-optical Kerr effect microscopy, and first-principles calculations, we present the direct evidence on the realization of the long-sought spontaneous ferromagnetism induced topological transition in soft ferromagnetic EuB_{6}. Explicitly, we reveal the topological transition is from Z_{2}=1 topological insulator in paramagnetic state to χ=1 magnetic topological semimetal in low temperature ferromagnetic state. Our results demonstrate that the simple band structure near the Fermi level and rich topological phases make EuB_{6} an ideal platform to study the topological phase physics.

Photophysics and Electronic Structure of Lateral Graphene/MoS2 and Metal/MoS2 Junctions.

Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. Here, we use spatially resolved photocurrent microscopy to demonstrate electronic uniformity at the epitaxial graphene/molybdenum disulfide (EG/MoS2) interface. A 10× larger photocurrent is extracted at the EG/MoS2 interface when compared to the metal (Ti/Au)/MoS2 interface. This is supported by semi-local density functional theory (DFT), which predicts the Schottky barrier at the EG/MoS2 interface to be ∼2× lower than that at Ti/MoS2. We provide a direct visualization of a 2D material Schottky barrier through combination of angle-resolved photoemission spectroscopy with spatial resolution selected to be ∼300 nm (nano-ARPES) and DFT calculations. A bending of ∼500 meV over a length scale of ∼2-3 μm in the valence band maximum of MoS2 is observed via nano-ARPES. We explicate a correlation between experimental demonstration and theoretical predictions of barriers at graphene/TMD interfaces. Spatially resolved photocurrent mapping allows for directly visualizing the uniformity of built-in electric fields at heterostructure interfaces, providing a guide for microscopic engineering of charge transport across heterointerfaces. This simple probe-based technique also speaks directly to the 2D synthesis community to elucidate electronic uniformity at domain boundaries alongside morphological uniformity over large areas.

Valence band inversion and spin-orbit effects in the electronic structure of monolayer GaSe

Two-dimensional monochalcogenides (MX) have been identified as a unique and promising class of layered materials in recent years. The valence band of single-layer MX, as predicted by theory, is inverted into a bow-shaped (often referred to as an inverted sombrero) and relatively flat dispersion, which is expected to give rise to strongly correlated effects. The inversion leads to an indirect band gap, which is consistent with photoluminescence (PL) experiments, but PL provides no direct evidence of the band inversion in the valence band. Here we demonstrate for a hexagonal MX crystal, gallium selenide (GaSe), using a combination of angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), that the valence band of monolayer (ML) GaSe exhibits a robust inversion of the valence dispersion at the \ensuremath{\Gamma} point forming a bow-shaped dispersion with a depth of 120 \ifmmode\pm\else\textpm\fi{} 10 meV between the double valence band maximum along the \ensuremath{\Gamma}$K$ direction. We also demonstrate that the deeper-lying bands detected in the ARPES spectrum are consistent with DFT calculations only if spin-orbit coupling is considered. The presented ARPES evidence that spin-orbit coupling leads to the splitting of two fourfold-degenerate states into four Kramers doublets is of significance for PL measurements, as the change in energy of the second highest valence state at the \ensuremath{\Gamma} point has a measurable effect on the PL energies in high-energy luminescence. We predict the optical absorption coefficients for the principal transitions in ML GaSe using a four-band $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model parametrized from first-principles theory with spin-orbit effects considered.

Surface Electronic Structure of Hybrid Organo Lead Bromide Perovskite Single Crystals

The electronic structure and band dispersion of methylammonium lead bromide, CH3NH3PbBr3, has been investigated through a combination of angle-resolved photoemission spectroscopy (ARPES) and inverse photoemission spectroscopy (IPES), as well as theoretical modeling based on density functional theory. The experimental band structures are consistent with the density functional calculations. The results demonstrate the presence of a dispersive valence band in MAPbBr3 that peaks at the M point of the surface Brillouin zone. The results also indicate that the surface termination of the CH3NH3PbBr3 is the methylammonium bromide (CH3NH3Br) layer. We find our results support models that predict a heavier hole effective mass in the region of −0.23 to −0.26 me, along the Γ (surface Brillouin center) to M point of the surface Brillouin zone. The surface appears to be n-type as a result of an excess of lead in the surface region.

Quadratic Fermi node in a 3D strongly correlated semimetal.

Strong spin–orbit coupling fosters exotic electronic states such as topological insulators and superconductors, but the combination of strong spin–orbit and strong electron–electron interactions is just beginning to be understood. Central to this emerging area are the 5d transition metal iridium oxides. Here, in the pyrochlore iridate Pr2Ir2O7, we identify a non-trivial state with a single-point Fermi node protected by cubic and time-reversal symmetries, using a combination of angle-resolved photoemission spectroscopy and first-principles calculations. Owing to its quadratic dispersion, the unique coincidence of four degenerate states at the Fermi energy, and strong Coulomb interactions, non-Fermi liquid behaviour is predicted, for which we observe some evidence. Our discovery implies that Pr2Ir2O7 is a parent state that can be manipulated to produce other strongly correlated topological phases, such as topological Mott insulator, Weyl semimetal, and quantum spin and anomalous Hall states.

Disentangling phonon and impurity interactions in δ-doped Si(001)

We present a study of the phonon and impurity interactions in a shallow two dimensional electron gas formed in Si(001). A highly conductive ultra-narrow n-type dopant δ-layer, which serves as a platform for quantum computation architecture, is formed and studied by angle resolved photoemission spectroscopy (ARPES) and temperature dependent nanoscale 4-point probe (4PP). The bandstructure of the δ-layer state is both measured and simulated. At 100 K, good agreement is only achieved by including interactions; electron-impurity scattering (W0 = 56 to 61 meV); and electron-phonon coupling (λ = 0.14 ± 0.04). These results are shown to be consistent with temperature dependent 4PP resistance measurements which indicate that at 100 K, ≈7∕8 of the measured resistance is due to impurity scattering with the remaining 1/8 coming from phonon interactions. In both resistance and bandstructure measurements, the impurity contribution exhibits a variability of ≈9% for nominally identical samples. The combination of ARPES and 4PP affords a thorough insight into the relevant contributions to electrical resistance in reduced dimensionality electronic platforms.

Formation of two-dimensional uranium silicide film and its electronic structure study

Uranium (U) deposition onto the Si (111) − 7×7 surface followed by annealing at 870K for 6min leads to the formation of 2D uranium silicide (USi1.67) film. The morphology and geometric structure have been studied by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED). The AlB2-type structure of USi1.67 epitaxial film is formed with a 1×1 periodicity. Morphologies of the crystalline USi1.67 film display triangular layered structures, and its electronic structure has been studied by a combination of angle-resolved photoemission spectroscopy (ARPES) and density functional theory calculations.

Graphene Epitaxy by Chemical Vapor Deposition on SiC

We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. (1) CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ∼1800 cm(2)/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.